It has been recognized for hundreds of years, that motor deficits following injury to the brain occur on the same side as a cerebellar lesion, but opposite the side of an injury to the cerebrum. The nature of the deficit also differs. Cerebral cortical injury causes muscle weakness or paralysis, while cerebellar lesions cause uncoordinated movements that are typically of the wrong size. Recognition of these specific motor signs caused great advances in the diagnosis and surgical treatment of such injury, but the personal and societal costs of treatment and rehabilitation following stroke or other injury remain tremendous.There are extensive interconnections between the cerebellum and the cerebral cortex. In fact, the primary motor cortex sends many more fibers to the cerebellum than it does to the spinal cord. Both structures have been studied extensively by means of recordings in behaving animals. Even so, conflicting ideas remain about the relation between these two areas, and the way in which they interact to produce and refine motor command signals. We propose to examine these relations by recording simultaneously in both the cerebellar nuclei (CN) and primary motor cortex (M1) in the awake, behaving monkey. In addition to paired recordings, we propose to use electrical micro-stimulation and averaging methods to examine the interconnections, and to micro-inject drugs during recording in order to distinguish convergent inputs. These measurements will be made during normal movement, and during motor learning, as the monkey adapts to novel forces imposed bya robotic device, that affect the trajectory of its movements.Simultaneous recordings provide far more than the ability to do two experiments at once. Recording simultaneously from select pairs of cells under literally identical experimental conditions provides a level of spatial and temporal precision with which to compare response properties that is not possible with single site recordings. More importantly, we will be able to associate these properties with the underlying functional connections between particular pairs of recording sites. This approach should greatly enhance our ability to understand the relation between M 1 and CN during the generation of motor command signals.